1. Field of the Invention
The present invention relates to exhaust gas purifying apparatus for purifying exhaust gas discharged from an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine mounted on an automotive vehicle or the like, it is necessary to purify discharged exhaust gas such as components of, for example, carbon monoxide (CO), nitrogen oxide (NOx), hydrocarbon (HC) and the like before being discharged into the atmosphere.
In particular, it is important to purify the unburnt gas components discharged in starting the internal combustion engine. In this case, in starting the internal combustion engine, an air/fuel ratio of mixture is set at a low air/fuel ratio (on an enriched side) in comparison with a stoichiometric air/fuel ratio in order to enhance startability of the internal combustion engine. However, since the temperature of the internal combustion engine is low and the combustion is unstable, a large amount of the unburnt gas components such as the unburnt hydrocarbon is discharged.
To meet this demand, an "engine exhaust gas purifying apparatus" described in Japanese Patent Application Laid-Open No. Hei 6-33747 is well known. In this apparatus, an adsorbent for adsorbing unburnt hydrocarbon (HC) contained in the exhaust gas below a predetermined temperature and for releasing the adsorbed unburnt hydrocarbon (HC) at a temperature equal to or higher than the predetermined temperature, is provided at an exhaust passage upstream of an exhaust gas purifying catalyst, and a heated catalyst is provided in the exhaust passage between the adsorbent and the catalyst.
In such an exhaust gas purifying apparatus, in the case where the engine is started in a cold condition so that the catalyst is kept under an inactive condition, the unburnt hydrocarbon (HC) discharged in starting the engine is adsorbed onto the adsorbent and current is fed to a heater of the heated catalyst to thereby activate the catalyst.
Thereafter, when the adsorbent is subjected to heat of the exhaust gas to reach a predetermined temperature, the unburnt hydrocarbon (HC) adsorbed on the adsorbent is released away from the adsorbent and is caused to flow into the heated catalyst. However, at this time, since the heated catalyst is activated by the heater, the above-described unburnt hydrocarbon (HC) is purified by the heated catalyst.
However, in the case where the above described exhaust gas purifying apparatus is applied to an internal combustion engine having a large displacement capacity, since a large amount of unburnt hydrocarbon (HC) is adsorbed on the adsorbent in starting the internal combustion engine, the large amount of unburnt hydrocarbon (HC) adsorbed on the adsorbent when a temperature of the adsorbent is elevated at a predetermined temperature is caused to enter into the heated catalyst at once. As a result, there is a possibility that the unburnt hydrocarbon would be discharged into the atmosphere without completely purifying the exhaust gas through the heated catalyst.